This invention relates to a process for purifying oxygen-containing recycle gas deriving from the ozonisation of mono or polyunsaturated organic compounds, such as hydrocarbons, contaminated with organic compounds. The invention also relates to a process for separating and recovering said organic compounds from said oxygen-containing gas. The invention further relates to a suitable apparatus for carrying out said separation of the organic compounds from the oxygen-containing gas, and the recovery of said organic compounds.
Further objects will be specified hereinafter.
Ozone is produced industrially by supplying energy by various methods to an oxygen-containing gas. In one of the most used methods, the gas is passed through a flat or cylindrically shaped interspace between two surfaces, one of which, generally covered by a dielectric, is subjected to a voltage of some effective KV, at a frequency of 50-3000 Hz and of a suitable wave form, whereas the other is connected to earth. One or both of these surfaces are cooled because the energy yield of the ozone formation from the oxygen is low, of the order of 10-20%, the rest being dissipated as heat.
The parameters which govern the energy yield and productivity of the ozone generators include the following:
temperature and pressure of the oxygen-containing gas; PA0 voltage, frequency and wave form; PA0 hydrodynamic and geometrical characteristics of the apparatus; PA0 ozone concentration in the outlet gas; PA0 composition of the gaseous mixture. PA0 condensation and formation of deposits on the electrodes; PA0 corrosion or alteration of the electrode surface; PA0 formation of other harmful, dangerous or otherwise undesirable chemical compounds on discharge; PA0 formation of explosive mixtures. PA0 (A) the opportunity to recover the entrained organic compounds, which are often of considerable quantity and/or value. In this respect, in this type of reaction very high gas flow rates are generally present because of the low ozone concentration, and in the contact reactor this gas comes into liquid-vapour equilibrium with the mixture contained in the reactor; PA0 (B) the need to finally obtain a gas in which the organic compounds are practically absent, and in any case less than 15 parts per million by volume. PA0 it does not allow recovery of the organic contaminating compounds, which are converted to carbon dioxide and water; PA0 it requires a considerable quantity of heat exchange between two gas streams, of which the stream containing the organic compounds is heated by the sensible heat of the gas after combustion. The temperature to which the gas is raised is generally around 400.degree. C.; PA0 after treatment, the gas has to be bled off to keep the carbon dioxide content constant, and dried to remove the water formed in order to return the dew point of the gas to below -50.degree. C.; PA0 the combustion of the organic substances is not total, so that certain of the drawbacks connected with the recycling of impure gas arise. PA0 (a) passage through a suitable apparatus in which the products contained in the gas in the form of fumes are separated, in particular peroxide products (these products, which are soluble in the organic acids and anhydrides contained in the gas and present on the filter element in the form of condensate can be recovered in this manner and recycled through the process); PA0 (b) countercurrent scrubbing with a polyoxyalkyleneglycol having a molecular weight of 200 to 10,000 and pretreated with an anhydride of a monocarboxylic acid containing 2 to 4 carbon atoms (hereinafter known as PAG I for simplicity); and/or PA0 (c) countercurrent scrubbing with a polyoxyalkyleneglycol having a molecular weight of 200 to about 10,000 (hereinafter known as PAG II for simplicity) or with an excess of a basic compound of an alkali metal in the form of an aqueous solution having a pH.gtoreq.8 (hereinafter known as solution X), PA0 because of the very high gas throughputs, the pressure drops on the gas side should be minimised to reduce costs, and because of this it is generally not convenient to use apparatus requiring heads of liquid to be traversed (for example plate columns of any type) PA0 because of the very small throughputs of liquid in contact with large throughputs of gas, it is at the same time advantageous to ensure good contact efficiency. For this reason, it is unadvisable to use packed columns, in which it would be impossible to completely wet the packing.
Of these parameters, the type and composition of the oxygen-containing gas used is very important, and in particular its humidity and the oxygen concentration.
In those cases in which relatively small ozone quantities are required, for example disinfection, drinking water and similar plants, it is generally preferred to use air in an open cycle. In this case, the air, which is taken from the outside, needs only to be filtered and carefully dried (dew point less than -50.degree. C.) before being fed to the ozone generator. The ozone-containing air is then fed to the reactor in which ozonisation is carried out, and it is then discharged to atmosphere. To be able to do this, it is necessary to remove the residual ozone by known means, and to eliminate any harmful substances which may have become absorbed during the reaction, so as to fall within the limits imposed by current regulations. This leads to a plant which is relatively simple in terms of its components, but which is of low production because of its high operating cost, the total energy consumption exceeding 20 KWh/Kg of ozone.
Where much greater quantities of ozone are required (such as in the ozonisation of organic compounds), either pure oxygen is used instead of the air, or a gas mixture having an oxygen content exceeding 20% by volume, and preferably exceeding 50% by volume, which, other conditions being equal, gives a productivity and yield which is more than doubled, and consequently ozone at approximately half the cost of that obtained using air. However, this assumes the nearly complete overall recovery of the gas, as the ozone is produced in a concentration of the order of 10-50 g/Nm.sup.3, and thus determines the need to operate in closed cycle. However, a further drawback arises when operating in closed cycle if the gas fed to the ozone generator contains organic substances (as in the case of gas deriving from the ozonisation of unsaturated hydrocarbons in the presence of organic acids and/or organio acid anhydrides) as these organic impurities increase during recycling, so leading to a reduction in the productivity of the ozone generator until it ceases to produce when the organic compound content is about 1% by volume.
Moreover, depending on the type of organic compounds concerned, further drawbacks can arise even if these compounds are present only in quantities of a few parts per million, these drawbacks including:
Manufacturers of ozone generating apparatus generally stipulate a maximum limit to the organic residue content of the gas, which is of the order of approximately 5-15 parts per million by volume.
Consequently, in chemical ozonisation processes in which (a) large ozone flow rate are required, and thus it is necessary to operate with gas in closed cycle in order to reduce the ozone cost, and in which (b) this gas comes into contact with organic substances in the reactor, the problem of purifying the gas with respect to the organic compounds arises. This problem has to be viewed from two aspects:
Even today, only a few processes use ozone in the chemical industry, due mainly to the difficulties which arise when the ozonisation gas is recycled for obvious process economy reasons.
As it has not as yet been possible to carry out processes based on absorbing organic contaminating compounds in solvent due to the fact that the solvent has its own vapour pressure and would therefore contaminate the gas, one of the known methods used industrially consists of passing the gas after ozonisation over a catalyst consisting of Mn, Cr or Cu oxides absorbed in alumina, in which the organic contaminating substances are subjected to combustion at the expense of the oxygen contained in the gas. However, this process has the following drawbacks: